This application claims the benefit of U.K. Application No. 9911433.2, filed May 17, 1999.
This invention relates in general to telephones, and more particularly to a circuit for eliminating click noise when activating a mute switch button.
Telephone microphones, particularly those in handsets and headsets, are often provided with a switch to xe2x80x9cmutexe2x80x9d the outgoing audio so that the user can talk with a local party without conveying information through the phone. The least expensive way to implement this mute function is to use the switch to disconnect the microphone from the telephone set, or to short circuit the microphone. When the switch is activated (or de-activated) a xe2x80x9cclickxe2x80x9d noise may be heard at the remote communication endpoint (and at the local endpoint) as a result of transients generated upon opening and closing of the switch.
Previously, the click noise has been eliminated by adding circuitry connected to the microphone and switch circuit which gently decreases (or increases) the signal from the microphone until the microphone is fully muted (or open). This additional circuitry adds cost (the circuitry which is in addition to the switch) and complexity to the product.
According to the present invention, a low cost click free muting circuit is provided which takes advantage of the delay in processing signals through a telephone codec circuit. The signal from the microphone passes through the mute switch (when closed) to an amplifier and from there to the codec for conversion to digital PCM. The digital PCM signal from the codec is then transmitted via a CPU to the system (e.g. PBX). According to the invention, a switch detect circuit is provided in parallel with the amplifier for detecting opening of the switch, in response to which a signal is sent to the CPU causing it to generate silence code while the switch is closed.